A compound semiconductor is a compound that is composed of at least two types of elements rather than one type of element such as silicon or germanium and operates as a semiconductor. Various types of compound semiconductors have been developed and are currently being used in various fields of industry. Typically, a compound semiconductor may be used in thermoelectric conversion elements using the Peltier Effect, light emitting devices using the photoelectric conversion effect, for example, light emitting diodes or laser diodes, fuel cells, and the like.
Particularly, a thermoelectric conversion element is used for thermoelectric conversion power generation or thermoelectric conversion cooling applications, and generally includes an N-type thermoelectric semiconductor and a P-type thermoelectric semiconductor electrically connected in series and thermally connected in parallel. The thermoelectric conversion power generation is a method which generates power by converting thermal energy to electrical energy using a thermoelectromotive force generated by creating a temperature difference in a thermoelectric conversion element. Also, the thermoelectric conversion cooling is a method which produces cooling by converting electrical energy to thermal energy using an effect that a temperature difference creates between both ends of a thermoelectric conversion element when a direct current flows through the both ends of the thermoelectric conversion element.
The energy conversion efficiency of the thermoelectric conversion element generally depends on a performance index value or ZT of a thermoelectric conversion material. Here, the ZT may be determined based on the Seebeck coefficient, electrical conductivity, and thermal conductivity, and as a ZT value increases, a thermoelectric conversion material has better performance.
Heretofore, many kinds of thermoelectric conversion materials have been proposed, but there is substantially no thermoelectric conversion material with sufficiently high thermoelectric conversion performance. In particular, thermoelectric conversion materials are applied to more and more fields, and temperature conditions may vary depending on their applied fields. However, since thermoelectric conversion materials may have different thermoelectric conversion performance depending on temperature, each thermoelectric conversion material needs to have optimized thermoelectric conversion performance suitable for its applied field. However, there is not yet proposed a thermoelectric conversion material with optimized performance for various and broad temperature ranges.